Hi,
A couple quick comments in addition to Don's excellent points.
First, always measure the radiator neck. This is a commonly replaced item, if/when a radiator is rebuilt. It's not uncommon to find a 3/4" neck on an older car that should have 1". Just measure from the inner flange to the top of the outer flange.
Next, TR3 used 4 lb. caps primarily to keep from bursting the core in the heater. The TR4 and later cars using increased pressure caps have different heater cores. Even so, I've seen those "bulging" threateningly.
*Be very careful putting a higher pressure cap on an earlier car!* There is a possibility you'll get wet feet (and carpets, etc).
It might be wise to temporarily disconnect the heater while testing the cap to see if the higher pressure makes much improvement cooling.
There is also a possiblity of head gasket or water pump seal problems. But, both are generally up to spec with later cars that used higher pressure in the cooling system.
Higher pressure caps are used on race cars, to raise the boiling point, just as has been described. But, most race cars don't have heaters.
Overall, I think there are better solutions to cooling problems and would not recommend a higher pressure rad cap. It's a risky experiment.
Alan Myers
San Jose, Calif.
'62 TR4 CT17602L